This invention relates to gas or vapor discharge lamps as for example, sodium vapor lamps, and specifically to a means for raising vapor pressure within the lamp to improve its performance.
In the prior art, sodium vapor lamps are typically constructed with an inner discharge tube containing the active material sodium, the discharge tube being surrounded by an outer envelope with a vacuum between the inner tube and the outer envelope. The outer envelope is coated with an infrared reflector. Thus, heat loss from the discharge tube is held at a minimum for the purpose of raising the temperature of the discharge tube and maintaining it at a suitable operating level of about 260.degree.-270.degree.C.
Prior to startup, in its cold condition, the sodium vapor pressure within the lamp is very low. As the electrical energy is applied, the tube warms up and the sodium vapor pressure increases. This startup process is not instantaneous but takes an appreciable time. Two considerations are important. First, the sodium must be vaporized by an initial arc established in an argon or neon atsmosphere. Second, in order to emit light of a constant and maximum intensity the discharge through the now-vaporized sodium must stablize. The preferred temperature of the discharge tube corresponding to such stablization is about 260.degree.-270.degree.C, considered to be the optimum operating temperature range of a sodium vapor discharge lamp.
The prior art vacuum space between discharge tube and outer envelope, and the infrared reflector on the outer envelope have been effective to maintain the lamp operating temperature after stablization. But they have not been very effective to shorten the time lag between startup and stablization. This time lag is of no consequence in applications such as street lighting. But, the length of time in warmup can be a problem as for example, when such a light source is used in a photocopying system. An extended wait after switching on a photocopier and before it is usable is unacceptable.